A method of treating electrically conductive nanoparticles using a dynamic processing electrochemical cell.

Full Text

METHOD OF TREATING NANOPARTICLES USING AN INTERMITTENTLY
PROCESSING ELECTROCHEMICAL CELL
TECHNICAL FIELD
[0001] The field to which the disclosure generally relates includes
methods of treating nanoparticles.
BACKGROUND
[0002] The electrochemical treatment of large quantities of
nanoparticles, including coating, stripping, oxidation, reduction, cleaning,
dealloying of nanoparticles and so on, has long been a technical barrier for
more extensive applications of this technique, in a range of applications
including, but not limited to, fuel cells, batteries, and heterogeneous catalysis.
Heretofore treatment of nanoparticles has resulted in extremely non-uniform
treatment of the nanoparticles.
SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION
[0003] One embodiment of the invention includes a method of treating
nanoparticles including using a container as a working electrode and
dynamically contacting the nanoparticles with the container so that the
nanoparticles are treated.
[0004] Other exemplary embodiments of the invention will become
apparent from the detailed description provided hereinafter. It should be
understood that the detailed description and specific examples, while

disclosing exemplary embodiments of the invention, are intended for purposes
of illustration only and are not intended to limit the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] Exemplary embodiments of the invention will become more fully
understood from the detailed description and the accompanying drawings,
wherein:
[0006] FIG. 1 illustrates an electrochemical cell useful in the treatment
of nanoparticles according to one embodiment of the invention.
[0007] FIG. 2 is an enlarged section view of a container including a
perforated material and a membrane wrapped around the same according to
one embodiment of the invention.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0008] The following description of the embodiment(s) is merely
exemplary in nature and is in no way intended to limit the invention, its
application, or uses.
[0009] FIG. 1 illustrates an electrochemical cell 10 according to one
embodiment of the invention. A container 16 such as, but not limited to, a
glassy carbon or platinum crucible, or a glass beaker covered on the inside
with platinum foil is used as the working electrode. Nanoparticles 20 to be
treated are immersed in a first electrolyte 12 to form a suspension 14. The
nanoparticles 20 may be made from any of a variety of electrically conductive
materials. For example, the nanoparticles 20 may include pyrolytic carbon
particles or strands of carbon particles fused together. The nanoparticles 20

may also include solid particles including metals or metal oxides. The
nanoparticles 20 may also include particles having hollow cores.
[0010] One embodiment of the invention includes using electrochemical
cell 10 including the container 16 as the working electrode and causing the
nanoparticles 20 being treated by the liquid electrolyte 12 to intermittently
contact the container 16. A means for flowing 36 the nanoparticles 20 in the
container 16 is provided such as a pump, a propeller or a magnetic stir bar.
The electrolyte 12 may be an aqueous acid solution including perchloric acid,
sulfuric acid, or phosphoric acid. Due to the convective vortex flow of the
suspension 14, the nanoparticles 20 intermittently contact an inner surface 17
of the container 16 working electrode. A counter electrode 24 which may be,
but is not limited to, a platinum gauze cylinder, may be placed in a second
container 46 which may include perforated material 40 such as
polytetrafluoroethylene (PTFE) or glass.
[0011] In one embodiment of the invention the electrochemical cell 10
includes a membrane 26, for example a proton conductive polymer electrolyte
membrane, that separates a working electrolyte compartment 34 from a
counter electrode compartment 35. As best seen in FIG. 2, the perforated
material 40 having a plurality of holes 42 formed therethrough may be
wrapped with the proton conductive polymer electrolyte membrane 26.
Alternatively, the membrane 26 may be placed inside of the perforated
container 40. In yet another embodiment, the perforated container 40 may be
eliminated and the membrane 26 may include a support material received
therein such as expanded PTFE.

[0012] The second container 46 is filled with a second electrolyte 18,
such as an aqueous acid solution including perchloric acid, sulfuric acid or
phosphoric acid. The electrolyte 18 is completely separated from the
suspension 14 in the working electrode side of the membrane 26. The
suspension 14 is in the working electrolyte compartment 34 and the second
electrolyte 18 is in the counter electrode compartment 35. A reference
electrode 28 may be placed in the suspension 14 close to the crucible wall. A
gas purging tube 30 may also be inserted in the suspension 14. Suitable
choices for the reference electrode 28 include, but are not limited to, a
silver/silver chloride electrode, a mercury/Calomel electrode, or a reversible
hydrogen electrode. The containers 16, 46 are sealed by a cover 32, which
may be polytetrafluoroethylene.
[0013] A potential is applied on the electrode 16, versus 28, by using
24 as the counter electrode. The device used to apply the potential may be,
for example, a potentiostat (not shown) to treat trie nanoparticles 20. This
arrangement may be utilized for coating, stripping, oxidation, reduction,
cleaning or dealloying the nanoparticles 20. As the suspension 14 is in
dynamic convection, the concentration of the reactant species 20 at the inner
container surface 17 approaches that in the bulk electrolyte, resulting in a very
small resistance in the electrolyte phase. The electronic resistance of the
container 16 is small as well. That both of these resistances are small
ensures a uniform treatment of the nanoparticles 20. The extension of
electrochemical treatment of the nanoparticles can be monitored by watching
the current change or the charge integration of the process at the treating
potential. For example, when coating a monolayer of copper on Pd/C

nanoparticles by under-potential-deposition (UPD) technique, which is an
important intermediate step of Pt monolayer to multilayer coating on Pd/C, the
potential of the working electrode is held at the UPD potential, and the
process is finished as the current approaches zero. By using this design, a
uniform electrical treatment of a very large volume of nanoparticles can be
achieved in a simple and neat way. The cell design combines some
advantages of a polymer electrolyte membrane fuel cell and some of a
conventional liquid electrolyte electrochemical cell. In the case when the
electrochemical reaction at the counter electrode is not the reverse reaction at
the working electrode (for example, when H2 or O2 evolution occurs at the
counter electrode), the design can essentially prevent the reaction products
(H2 or O2) from diffusing to the working electrode. As the nanoparticles are
immersed in the liquid electrolyte and intermittently contact the working
electrode, all of the nanoparticles can eventually be uniformly treated and can
be easily washed out after the treatment. Neither of these features can be
achieved for the catalyst layer in polymer electrolyte membrane fuel cells, in
which the catalyst layer is mixed with a solid ionomer phase.
[0014] In various embodiments, the polymer electrolyte membrane 26
may include a variety of different types of membranes. The polymer
electrolyte membrane 26 useful in various embodiments of the invention may
be an ion-conductive material. Examples of suitable membranes are
disclosed in U. S. Patent Nos. 4,272,353 and 3,134,689, and in the Journal of
Power Sources, Volume 28 (1990), pages 367-387. Such membranes are
also known as ion exchange resin membranes. The resins include ionic
groups in their polymeric structure; one ionic component for which is fixed or

retained by the polymeric matrix and at least one other ionic component being
a mobile replaceable ion electrostatically associated with the fixed
component. The ability of the mobile ion to be replaced under appropriate
conditions with other ions imparts ion exchange characteristics to these
materials.
[0015] The ion exchange resins can be prepared by polymerizing a
mixture of ingredients, one of which contains an ionic constituent. One broad
class of cationic exchange, proton conductive resins is the so-called sulfonic
acid cationic exchange resin. In the sulfonic acid membranes, the cationic
exchange groups are sulfonic acid groups which are attached to the polymer
backbone.
[0016] The formation of these ion exchange resins into membranes or
chutes is well-known to those skilled in the art. The preferred type is
perfluorinated sulfonic acid polymer electrolyte in which the entire membrane
structure has ionic exchange characteristics. These membranes are
commercially available, and a typical example of a commercial sulfonic
perfluorocarbon proton conductive membrane is sold by E. I. DuPont D
Nemours & Company under the trade designation NAFION. Other such
membranes are available from Asahi Glass and Asahi Chemical Company.
The use of other types of membranes, such as, but not limited to,
perfluorinated cation-exchange membranes, hydrocarbon based cation-
exchange membranes as well as anion-exchange membranes are also within
the scope of the invention.
[0017] The electrochemical cell 10 may be used to coat nanoparticles
20 with a catalyst such as platinum to provide a plurality of supported catalyst

particles. The supported catalyst particles may be combined with an ionomer
which may be the same as the material for the above described membrane
26. The supported catalyst particles and ionomer may be applied to both
faces of a polymer electrolyte membrane of a fuel cell. The supported
catalyst particles and ionomer may alternatively be applied to a fuel cell gas
diffusion media layer or onto a decal backing for later application as desired.
[0018] The above description of embodiments of the invention is merely
exemplary in nature and, thus, variations thereof are not to be regarded as a
departure from the spirit and scope of the invention.

CLAIMS
What is claimed is:
1. A method comprising:
electrochemically treating electrically conductive nanoparticles
comprising providing an electrochemical cell comprising a container as a
working electrode, providing a suspension in the container comprising the
nanoparticles and a liquid electrolyte, and intermittently contacting the
nanoparticles with an inner surface of the container to treat the nanoparticles.
2. A method as set forth in claim 1 wherein the intermittently
contacting the nanoparticles with the inner surface of the container comprises
stirring the suspension.
3. A method as set forth in claim 2 wherein the stirring the
suspension comprises rotating a magnetic stirring bar.
4. A method as set forth in claim 1 wherein the electrochemical cell
includes a counter electrode.
5. A method as set forth in claim 4 wherein the treating comprises
applying a potential between the working electrode and the counter electrode.
6. A method as set forth in claim 4 wherein the counter electrode
comprises a gauze.

7. A method as set forth in claim 4 wherein the counter electrode
comprises a gauze comprising platinum.
8. A method as set forth in claim 4 further comprising a polymer
electrolyte membrane separating the working electrode from the counter
electrode to provide a working electrode compartment and a counter
electrode compartment.
9. A method as set forth in claim 8 further comprising a gas purge
tube received in the first container and outside of the cylinder.
10. A method as set forth in claim 8 wherein the electrochemical cell
further comprises a reference electrode received in the first container.
11. A method as set forth in claim 1 wherein the nanoparticles
comprise at least one of Pt, Pt alloy, a rioble metal, a metal alloy, a metal
oxide, or carbon nanoparticles.
12. A method comprising:
providing an electrochemical cell comprising a first container as
a working electrode, a suspension received in the first container comprising a
plurality of nanoparticles and a first liquid electrolyte, a second container
received in the first container, and a second liquid electrolyte received in the
second container, and a counter electrode received in the second container,

the second container comprising a cylinder comprising polytetrafluoroethylene
or glass, and a membrane wrapped around the cylinder, and wherein the
membrane completely separates the suspension contained in the first
container from the second electrolyte contained in the second container, a
reference electrode received in the first container, applying a potential
between the electrodes, and stirring the suspension to cause the
nanoparticles to intermittently contact an inner surface of the first container
and thereby be treated.
13. A method as set forth in claim 12 wherein the counter electrode
comprises platinum gauze.
14. A method as set forth in claim 12 wherein the electrochemical
cell further includes a magnetic stirring bar in the first container and wherein
the stirring the suspension comprises rotating the magnetic stirring bar.
15. A method as set forth in claim 12 further comprising a cover
over the first and second containers.
16. A method as set forth in claim 12 further comprising a gas
purging tube received in the first container.
17. A method as set forth in claim 12 wherein the nanoparticles
comprise at least one of Pt, Pt alloy, a noble metal, a metal alloy, a metal
oxide, or carbon nanoparticles.

A method of treating electrically conductive nanoparticles using a dynamic processing electrochemical cell.